This invention relates to a transparency sheet, particularly a transparency sheet for use in an ink jet printer.
U.S. patent application Ser. No. 07/715,063 filed Jun. 12, 1991 (hereinafter referred to as "the prior application") discloses an ink jet printer for forming an image on an image-receiving sheet. The disclosure of the prior application is hereby incorporated by reference herein.
The ink jet printer described in the prior application comprises a drum for supporting the image-receiving sheet, which is typically of paper, as ink drops are deposited on the sheet to form the desired image. A stepper motor is connected drivingly to the drum for rotating the drum, and an ink jet head is mounted on a carriage that extends parallel to the axis of the drum. The carriage is movable longitudinally of the drum, and as the carriage moves, the ink jet head is energized to deposit ink drops on the sheet.
In order to load the printer with a sheet, the drum is rotated to a load position and a clamp at the periphery of the drum is opened. The sheet is fed substantially tangentially towards the clamp, and the clamp is closed and thereby grips the leading edge portion of the sheet. The drum is rotated through about 70.degree. from the load position to a print start position, in which the clamp is close to the path of the ink jet head. During printing, the carriage is driven axially of the drum while the ink jet head scans the sheet and ejects ink drops onto the sheet, and the drum is rotated stepwise at the end of each scan of the ink jet head. At the end of printing, the drum is rotated in the same direction until the trailing edge of the sheet is at an exit position, and the drum is then rotated in the reverse direction, introducing the trailing edge of the sheet into an exit path. The clamp is opened and releases the leading edge portion of the sheet and the sheet is discharged from the printer.
In a practical implementation of the printer described in the prior application, the printer includes two reflective sensors for sensing the presence of a paper sheet on the drum. Each sensor includes a light emitter positioned to direct a light beam towards the drum and a light detector for receiving reflected light. The surface of the drum is highly reflective and reflects light in a specular fashion, whereas paper reflects light in a diffuse manner. Therefore, when paper is interposed between a sensor and the surface of the drum, the sensor provides an output signal having a lower level than when there is no paper present. It is essential to proper operation of the sensors that the surface of the drum remain highly reflective, and therefore it is important that no ink be applied to the drum. Further, if ink is applied to the drum there is an increased possibility of sheet misfeeds, since the surface of the drum then has different mechanical properties from when it is clean.
One of the sensors (the Y-axis sensor) is located stationarily between the load position and the print start position. When the drum rotates from the load position to the print start position, the output of the Y-axis sensor indicates whether the sheet has been successfully loaded. If the sheet has been successfully loaded, the output of the Y-axis sensor allows further operation to take place. If the output signal of the Y-axis sensor indicates that the sheet has not been successfully loaded, the printing operation aborts and another attempt is made to load a sheet. Further, towards the end of the printing operation the Y-axis sensor detects the trailing edge of the sheet and its output ensures that printing ceases before the trailing edge reaches the printing position.
The other sensor (the X-axis sensor) is mounted on the traversing carriage with the ink jet head. When the drum has rotated to the start print position, but before printing takes place, the carriage is driven lengthwise of the drum and the X-axis sensor scans the drum. The output signal of the X-axis sensor during this probe scan confirms that there is an image-receiving sheet at the printing position and also provides information regarding the location of the edges of the sheet along the axis of the drum. This information is used to ensure that ink is applied only between the two edges of the sheet that extend longitudinally with respect to the path of movement of the sheet. The X-axis sensor continues to sense the longitudinal edges of the sheet throughout the printing operation so that if, for example, the sheet becomes narrower from the leading edge towards the trailing edge, the image will be progressively cropped to ensure that no ink is applied to the drum.
It is frequently desirable to form an image on a transparency sheet to allow the image to be displayed using an overhead projector. Transparency sheets for overhead projectors are typically made of polyester film. A transparency sheet cannot normally be printed successfully in an ink jet printer simply by feeding the transparency sheet into the printer instead of a paper sheet. For example, if a transparency sheet were introduced into the printer described in the prior application without making changes to accommodate the difference between the optical properties of a transparency sheet and those of a paper sheet, the Y-axis sensor would not detect the presence of the sheet and accordingly the printer would not recognize that the sheet had been loaded and printing could not take place. In addition, the mechanical properties of a transparency sheet made of polyester film are quite different from those of paper of the kind normally used in an ink jet printer, particularly with respect to the coefficient of friction, and this can cause difficulties in handling a transparency sheet in an ink jet printer.